This invention relates to cable television systems, and more particularly, to monitoring distortion in a cable television system.
Cable television (CATV) systems provide content to subscribers, typically by routing signals from a satellite to a subscriber utilizing communication systems such as fiber optic cables and/or coaxial cables. FIG. 1 shows an example of a cable television system 10 including a satellite 12 transmitting signals to a headend 14 that receives and demodulates the signals to baseband. The baseband signal is transmitted over the communication system after conversion to an optical signal or a radio frequency. As illustrated, a fiber optic system 16 transports the optical signal to a hub 18 and then onto a node 20. Amplifiers 22 are normally utilized in the system to increase the signal. Inserted in the communications system are taps 24 to tap off the signals to subscribers 26.
Conventionally, distortion in CATV systems is monitored by utilizing a pilot signal. A pilot signal of a specific frequency is transmitted through the network and the level of this pilot signal is detected at the input and output of the amplifier, such as amplifier 22 shown in FIG. 1. By examining these levels, a determination is made whether the CATV signal level is acceptable. Based on this, an excessive signal level can be detected which indirectly indicates an increase in signal distortion.
In the current method, if a distortion results due to disturbances at a frequency different from the pilot frequency, it may not be possible to detect that distortion. Typically, system amplifier hybrids are driven near the maximum signal point (i.e. the crash point) to obtain the highest output level. As a result, a small change in output level can cause a large change in distortion. Thus, to maintain peak performance of an overall system requires close monitoring of distortion.
Additionally, if distortion is discovered, it is measured at the output of the system. Currently, the location or component that originated the distortion cannot be determined.
Another method currently employed to monitor distortion is by the use of a spectrum analyzer placed at the end of the transmission. However, with this approach diagnosing the source of the problem has proven difficult.
Accordingly, a need exists for systems and methods for monitoring distortion to detect an abnormal increase of signal distortion occurring anywhere in the transmission system while system output.
This invention addresses the prior problems and provides for systems and methods for a distortion monitoring system adapted to detect an abnormal amount of signal distortion occurring anywhere in a transmission system.
The increase of channel loading in a CATV system, in particular, the addition of digital channels at the upper spectrum as well as the transmission of video signals over fiber links, necessitate the need for a distortion monitoring system to detect an abnormal increase of signal distortion occurring in the CATV system.
This invention includes a system amplifier section for amplifying and coupling an input signal to provide first and second outputs and a distortion monitoring section for processing the second output. The amount of distortion present is represented by an output from the distortion monitoring section of the system. The distortion monitoring section can include an envelope detector for receiving the second output signal, a lowpass filter coupled to the envelope detector for cutting off frequencies below the Nyquist rate, a voltage amplifier coupled to the lowpass filter for amplifying the signal, an analog to digital converter coupled to the voltage amplifier for digitizing the signal, a memory buffer coupled to the analog to digital converter for capturing digital data from the analog to digital converter, and a digital signal processor for analyzing the signal and computing distortion.
In another embodiment, the distortion monitoring section encompasses the envelope detector, lowpass filter, voltage amplifier, an analog to digital converter, memory buffer, digital signal processor, and a digital to analog converter coupled to the digital signal processor for converting the digital signal to analog and providing an analog output signal indicating distortion.
The digital signal processor can also encompass a spectral estimation algorithm such as a Fast Fourier Transform or the Maximum Entropy Method, or the All Poles Method for isolating the desired converted signal and for indicating the power of the converted signal, and an averager for averaging the amplitude of the beats around the converted signal. In another embodiment, the digital signal processor includes a digital filter coupled to the memory buffer and adapted to provide a very narrow digital band pass filter centered at the nominal beat, a squarer coupled to the digital filter for squaring the output, a summer coupled to the squarer for summing the output, and an averager coupled to the summer for averaging the output and providing the beat level and distortion level of the output. In yet another embodiment, the digital signal processor includes an on chip memory.
This invention takes advantage of the intermodulation products that occur in the CATV transmission system. The detection units are placed strategically throughout the transmission system to monitor distortion from key elements of the transmission system. For example, detection units can immediately follow RF amplifiers and optical receivers. In this way, the invention can be used to monitor the distortion introduced by the amplifiers, lasers, and optical amplifiers in the networks. Thus, this invention can monitor distortion wherever an electrical signal exists.
Objects of the invention include:
To provide a system that monitors system distortion without utilizing a pilot signal.
To provide a system that takes advantage of the intermodulation beats to monitor distortion.
To provide for use of a digital signal processing (DSP) arrangement which makes the system flexible for a variety of channel plans, such as, for example, Phase Alternating Line (PAL) system and Comite European de Normalization Electrique (CENLEC).
To provide a system where the frequency to be monitored is selectively determined by software, which does not require tuning, which simplifies the manufacturing process.
To provide an output signal in digital or analog form which can be transported anywhere in the transmission system.
To provide a system capable of detecting and selecting beats at low frequencies which enables sampling to be carried out at a low rate.
To provide an easily manufactured system.
Other objects and advantages of this invention will become apparent to those skilled in the art upon review of this document, the Figures and the claims.